Search for double $\beta$-decay modes of $^{64}$Zn using purified zinc

TL;DR

This study develops a highly pure zinc material to improve the search for rare double beta-decay processes in zinc-64, setting new lower limits on their half-lives through sensitive detection techniques.

Contribution

The paper introduces an innovative high-purity zinc production method and demonstrates its application in setting new experimental limits on double beta-decay modes of zinc-64.

Findings

Produced zinc with 99.999% purity and low contamination levels.

Set the most stringent lower limits on half-lives for specific double beta-decay modes.

Validated the effectiveness of the purification process for rare event searches.

Abstract

The production of ultra-pure raw material is a crucial step to ensure the required background level in rare event searches. In this work, we establish an innovative technique developed to produce high-purity (99.999%) granular zinc. We demonstrate the effectiveness of the refining procedure by measuring the internal contaminations of the purified zinc with a high-purity germanium detector at the Laboratori Nazionali del Gran Sasso. The total activity of cosmogenic activated nuclides is measured at the level of a few mBq/kg, as well as limits on naturally occurring radionuclides are set to less than mBq/kg. The excellent radiopurity of the zinc sample allows us to search for electron capture with positron emission and neutrinoless double electron capture of $^{64}$Zn, setting the currently most stringent lower limits on their half-lives, $T_{1/2}^{\varepsilon\beta^+} > 2.7 \times 10^{21}\text{yr}$ (90% C.I.), and $T_{1/2}^{0\nu2\varepsilon}> 2.6 \times 10^{21}\text{yr}$ (90% C.I.), respectively.